Footage has been released of Nikolas Cruz asking police to kill him shortly after he was arrested following the massacre at a Florida high school.

Cruz faces 17 counts of premeditated murder and 17 of attempted murder after the 14 February mass shooting at Marjory Stoneman Douglas High in Parkland, Florida.

Police have released nearly 12 hours of footage, some showing Cruz being questioned by an officer and some showing him alone in the interrogation room.

While alone, the 19-year-old is seen staring into the camera, pointing his fingers at his head in a gun-like gesture and saying "kill me".

He describes himself as deserving to die because he is "worthless".

He also cries, punches himself in the face and scratches his arm with a small object he appears to have picked up off the floor.

Slouching in his chair and wearing hospital clothes, Cruz is regularly asked by the officer to speak louder and at one point his brother Zachary, 18, tries to speak with him, asking what their mother would have thought.

Image:Nikolas Cruz's brother Zachary speaks to him after his interview with police

Cruz, a former pupil of Marjory Stoneman Douglas High, tells the officer that he has heard the demonic voice inside his head for years and that it had told him to "burn, kill, destroy".

The voice had begun after Cruz's father died about 15 years ago and it worsened after his mother died in November, he said, adding that he would set fires in a pit and kill wild birds.

Cruz also told the detective that he was depressed, lonely, and had tried to kill himself.

Image:Seventeen people were killed at the high school

On Wednesday, the sheriff leading the state commission investigating the massacre said Cruz's behaviour before the shooting had been a "roller coaster" - there would be stretches of good conduct and then he would deteriorate.

Pinellas County Sheriff Bob Gualtieri said this had made it difficult for school officials to handle him and he spent years being moved between regular schooling and campuses for children with emotional and behavioural problems.

At times, Cruz also needed an escort while at school because of his behaviour, the sheriff said.

A threat assessment was completed on the teenager in 2016 and he was expelled from school about five months later.